Rachael was a Nexus-7 android. Evidently they had an elongated lifespan as well as the capacity to bear live young. Note that Wallace is very keen to learn how the latter task was accomplished (to the point of committing multiple crimes including kidnapping and torture) but natural lifespans were incorporated into Tyrell's Nexus-8 and Wallace's Nexus-9 models.
NIANDER WALLACE: I cannot breed them. I have tried, so help me. Tyrell’s final puzzle in adamantine chains and penal Fire.
We need more Replicants than can ever be assembled. Millions so we can be trillions. More. Worlds beyond worlds, diamond shores. We could storm Eden and retake her...
His last trick. Procreation. Perfected... then lost. There is a child. Finally unearthed. Bring it to me. Bring it to me. Bring it to me.
And from the Theatrical version voiceover of Blade Runner:
Deckard: Gaff had been there, and let her live. Four years, he figured. He was wrong. Tyrell had told me Rachael was special: no termination date. I didn't know how long we had together. Who does?